Recent growing interest in silicon photonics owes to the well-established CMOS (Complementary-Metal-Oxide-Semiconductor) manufacturing base for silicon-based chips and the need for photonics-based energy efficient technology. In addition to low-cost foundry base, silicon is transparent to light transmission at wavelengths used for telecommunication and data communication and has a high refractive index, allowing confinement of modes in sub-micron dimensions. Hence, silicon is a good candidate for energy-efficient micro- and nano-photonic applications.
However, one disadvantage of silicon is that it is an indirect bandgap material. Hence, silicon cannot be used to emit light. Therefore, heterogeneous integration of direct bandgap material such as III-V-based alloys on silicon via direct bonding or interlayer bonding is one way of realizing laser-on-silicon. In heterogeneously integrated hybrid III-V/SOI-based lasers, the optical field is generated in the III-V region and is coupled to the silicon layer. In the silicon layer, the passive devices are patterned to provide different functionalities such as waveguiding, reflection, filtering or modulation to the optical field. The silicon processing steps for patterning passive devices together with the heterogeneous integration of laser-on-silicon extend the scope of functionalities and complexity of the silicon photonics chip. This technique has already been used to realize various types of lasers such as Fabry-Perot laser, ring/disk lasers, multi-wavelength laser, tunable laser, and grating laser.
However, conventional lasers suffer from various deficiencies or problems. For example, there exists a conventional tunable laser which utilizes an electrically-pumped active medium aligned to mechanically-controlled gratings via lens system. By tuning the orientation of the gratings, the wavelength of the reflected optical field into the medium is tuned. This is because the beam sees varying periodicity of grating as a function of its orientation angle. Hence the wavelength with maximum reflection or the laser resonant wavelength changes, resulting in the laser emission at varying wavelength that is dependent on the orientation of the grating mirror. However, some disadvantages associated with this conventional tunable laser are: 1) since it depends on external diffraction grating and lens, it is not compact and integrate-able, 2) mechanical tuning is relatively slow (few milliseconds to seconds), 3) optical losses depends on strict alignment between lens and grating laser, and 4) packaging cost is high because of the usage of discrete elements such as lens and gratings.
There also exists an integrated version of grating laser-on-silicon that requires an active III-V medium heterogeneously integrated on silicon that is patterned with waveguide and gratings. The evanescently coupled optical field generated in electrically-pumped III-V-based active medium was guided in the silicon waveguide to the gratings. Although gratings reflected the optical field at a particular wavelength, since gratings was fabricated in silicon, it was not possible to tune the grating orientation. In addition, the footprint of the device was about 1 mm2 which is relatively large. Therefore, grating laser may not be a good solution for integrated tunable lasers-on-silicon.
There has also been disclosed a tunable laser-on-silicon. A silicon-based micro-ring was integrated in the cavity along the round-trip path of the optical field, in such a way that it filtered and only allowed the lasing wavelength to complete the round trip path. The tuning of the lasing wavelength was realized by tuning the filter wavelength of the ring via heating. A long active medium was realized through heterogeneous integration of III-V on SOI via interlayer bonding. The structure is relatively compact, but requires distributed Bragg reflectors (DBRs) for unidirectional laser emission. Further, since the heterogeneous integration is based on interlayer bonding, a relatively complex 3-layer coupling mechanism of optical field between the SOI and the III-V-based active layers was adopted which results in increased fabrication complexity.
A need therefore exists to provide an integrated laser that seeks to overcome, or at least ameliorate, one or more of the deficiencies of the conventional lasers mentioned above. It is against this background that the present invention has been developed.